The recycling of thermoplastic materials is desirable for both economic and environmental reasons. Heretofore, extruded articles have been formed of recycled plastic material generally by one of two processes. In the first process, measured amounts of solid thermoplastic material are reduced and mixed in a Bandbury or other batch-type mixer. The mixture may also be heated while in the mixer, either to a temperature near or at the melting temperature of the thermoplastic material, and then transferred to an extruder. The premeasured batch of material is then forced through a die by the extruder. This process is inherently a batch process wherein a predetermined amount of thermoplastic material, usually from about 14 kg to 30 kg (250 to 450 lbs), is processed at a time. The batch mixes are difficult to transfer to, and match the output requirements of the forming equipment, i.e., the extruder. Also, batch mixers of this type are expensive to purchase and service.
In the second process, reclaimed thermoplastic particles, in solid form, are fed directly into an extruder and the solid particles melted, pressurized, and extruded through a die. However, this process is inherently slow because the thermoplastic material must be held in the extruder for a length of time sufficient for the material to heated to its melting point. Thus, throughput, or the output production rate of the fluid plastic material, is quite limited, causing the cost of end products made by this process very expensive. Also, some materials such as polyvinyl chloride, when heated in an extruder, may cause physical damage to the machine as a result of chemical reaction. Moreover, because extruders are built to very close tolerances and have small internal clearances between the screw conveyor and inner case, contaminates in scrap plastic material can severely damage the extruder. The increased downtown, repair and service requirements further exacerbate the economic disadvantages of using an extruder for the direct conversion of recyclable scrap thermoplastic materials to extruded shapes.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a process whereby recyclable scrap thermoplastic material can be simultaneously mixed, heated, melted, prior to delivering as a continuous molten stream of material to an extruder. Furthermore, it is desirable that the rate of delivery of the molten stream to the extruder be selectively variable to match the processing requirements of the extruder. It is also desirable to have an apparatus wherein the recyclable thermoplastic material can be progressively heated simultaneously with mixing and conveying the material from a position at which it is received as solid particles to an output position at which it is an essentially homogeneous flowable mass.